1. Field of the Invention
The present invention relates to ladar systems. More specifically, the present invention relates to systems and methods for correcting phase errors in coherent ladar systems.
2. Description of the Related Art
Coherent ladar applications and techniques are beginning to be considered for various applications including commercial and military. These include high resolution velocity measurements, remote vibration measurements, optical synthetic aperture implementations, and others. High resolution coherent ladars typically require very stable laser oscillators and well controlled modulation waveforms. However, the increasing demand for fidelity, resolution, and long range of operation make the component requirements impossible to meet using conventional ladar systems.
A ladar system typically includes a laser transmitter that produces a laser signal which is transmitted towards a target scene. The laser signal reflects off the target, producing a laser return that is subsequently detected by the ladar system. The intensity of the return signal and the round trip delay from transmission to detection yield the distance (range) to the target. Coherent ladar systems transmit a laser signal having a predetermined phase and frequency. Knowledge of the spectral characteristics of the transmitted signal enables coherent ladar systems to record additional information about the scene, such as target movement. The velocity of a target can be determined from the frequency spectrum of the laser return.
A coherent ladar system typically includes a receiver detector that is illuminated by the laser return and a local oscillator (LO) reference signal. The mixing of the laser return with an LO signal allows the system to sense the phase and frequency of the return signal and also to amplify it to maintain the signal-to-noiseratio (SNR) of the return. The detector outputs a cross product of the laser return and LO optical fields. The desired information about the target is contained in the portion of the detector's output that oscillates at the frequency difference between the LO reference signal and the laser return.
In order to obtain high fidelity information, particularly in high coherence systems such as vibration sensitivity or Doppler sensitivity for synthetic aperture, the quality or fidelity of the transmitted signal must be very high. Laser transmitters, as other transmitters, tend to drift frequency in time, creating a limitation on the signal fidelity that can be achieved. In addition, large time-bandwidth modulated transmitted waveforms also require a high degree of phase control and ultimately also limit the return signal fidelity that can be achieved. Conventional ladar systems cannot meet the component stability requirements (laser transmitter stability and modulator linearity) necessary for high fidelity, high resolution, long range applications.
Hence, a need exists in the art for an improved ladar system having reduced component stability requirements than prior ladar systems.